Process for making furanones

ABSTRACT

This deals with a process for making furanones having the formula: ##STR1## wherein R represents a hydrogen atom or the methyl or ethyl group.

These furanones are known flavoring substances.

The furanones I are made by cleaving cyanohydrin from the followingnovel compounds: ##STR2##

The novel compounds II are made by oxidizing the novel compounds:##STR3##

FIELD OF THE INVENTION

This invention relates to novel cyano compounds and to furanones.

SUMMARY OF THE INVENTION

See the Abstract of The Invention, supra.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The furanones manufactured in accordance with the present invention arecompounds of the general formula: ##STR4## wherein R represents ahydrogen atom or the methyl or ethyl group.

It will be appreciated that in the formulae herein the R-symbols canhave the same significance or a different significance.

The process provided by the present invention for the manufacture of thecompounds of formula I comprises subjecting a compound of the generalformula: ##STR5## wherein R has the significance given earlier, to thecyanohydrin cleavage.

The cyanohydrin cleavage can be carried out thermally or can bebase-catalysed.

In the thermal cyanohydrin cleavage, a compound of formula II isconveniently heated to a temperature of ca 50° C. to 400° C., especiallyto ca 80°-250° C. Addition of an acid, e.g. H₂ SO₄ is advantageous.

In the base-catalysed cyanohydrin cleavage, a compound of formula II istreated with a base, the base being conveniently used in catalyticamounts (e.g. with 1/1000-1/10 equivalents) or, however, also in larger(e.g. molar) amounts.

The nature of the base is not critical. Examples of bases which can beused are inorganic bases such as alkali metal hydroxides (e.g. sodiumhydroxide), alkaline earth metal hydroxides (e.g. calcium hydroxide andmagnesium hydroxide), alkali metal carbonates (e.g. sodium carbonate andpotassium carbonate), alkali metal bicarbonates (e.g. sodiumbicarbonate), ammonia, other basic salts (e.g. sodium phosphate,potassium hydrogen phosphate and borax), basic buffer systems (e.g.sodium bicarbonate/sodium carbonate, potassium hydrogenphosphate/potassium phosphate etc.), organic bases (e.g. amines such astriethylamine, pyridine, morpholine etc.), salts of organic acids withstrong bases (e.g. sodium acetate, formate, oxalate, citrate andlactate) or basic ion exchangers (e.g. Amberlite IRA 400, Dowex 2,etc.).

The basic-catalysed cyanohydrin cleavage can be carried out in the gasphase or in the liquid phase. The presence of a solvent is notnecessary, but is convenient.

The base-catalysed cyanohydrin cleavage is preferably carried out at atemperature of 50°-200° C., especially ca 100° C.

The nature of the solvent is not critical. A polar solvent such aswater, ammonia or an alcohol, or an apolar solvent such as toluene,benzene, ether, petroleum ether etc. can be used.

Preferred systems are basic ion exchangers in the OH.sup.⊖ form/water,or salts of organic acids such as sodium oxalate/water orpyridine/toluene at a temperature of ca 100° C.

The compounds of formula II are novel and also form part of the presentinvention.

The compounds of formula II can advantageously be prepared by oxidisinga compound of the general formula ##STR6## wherein R has thesignificance given earlier.

Especially suitable oxidising agents are alkali metal caroates (e.g.KHSO₅). The preferred oxidising agent is "Caroat" (Trade Mark) (KHSO₅containing [small amounts of] KHSO₄ and K₂ SO₄).

The caroate is conveniently used in an amount of 1-2.5 equivalents,especially 1.1-1.5 equivalents.

The oxidation is preferably carried out in a polar solvent such aswater, an alcohol, acetone or acetonitrile or in a mixture of suchsolvents.

The pH of the medium in which the oxidation is carried out convenientlyamounts to ca 3-11, such as can be generated by appropriate buffersystems of the carbonate, phosphate, citrate, borate, NH₃ /NH₄ ⁺ oroxalate type in a manner known per se.

The oxidation can be carried out at a temperature of, for example,between -10° C. and 60° C., preferably between 0° C. and 20° C.

The compounds of formula III are novel and also form part of the presentinvention.

The compounds of formula III can advantageously be obtained by reactinga nitrile of the general formula

    R--CH═CH--CN                                           (IV)

with an ester of the general formula ##STR7## wherein R has thesignificance given earlier and R₁ represents a lower alkyl group.

The reaction of a nitrile of formula IV with an ester of formula V isconveniently carried out at an elevated temperature; for example, at40°-100° C. and especially at about 60° C. The reaction can also becarried out at a lower temperature (e.g. at room temperature). However,at this temperature the formation of byproducts is observed. Thesebyproducts must be removed (e.g. by extraction in a weak basic medium)during the working-up of the compound of formula III.

The molar ratio of nitrile of formula IV to ester of formula Vpreferably amounts to 1:1.

The reaction is conveniently carried out in the presence of a base andin a solvent.

As the base there are conveniently used 1-2 equivalents, especially1-1.3 equivalents, of a strong base; for example, a hydride such assodium hydride, an amide such as potassium amide, lithiumdiisopropylamide etc., a hydroxide such as sodium hydroxide, analcoholate such as sodium ethylate or potassium isopropylate or a metalsuch as sodium or potassium.

As the solvent there is especially used a polar, preferably aprotic,solvent.

Examples of such solvents are ethers such as tetrahydrofuran, dioxan,diglyme, diethyl ether and diisopropyl ether, nitro compounds such asnitromethane and nitrobenzene, nitriles such as acetonitrile,dimethylformamide, dimethyl sulphoxide etc. Ethers are especiallypreferred.

Examples of protic solvents are alcohols such as tert.butanol andisopropanol.

The compounds of formula I are known. They are useful as flavouringsubstances.

The following Examples illustrate the present invention:

EXAMPLE 1

(a) 130 g (1.1 mol) of ethyl lactate are added dropwise to 24 g (1 mol)of sodium hydride in 500 ml of tetrahydrofuran. 73.7 g (1.1 mol) ofcrotonic acid nitrile in 60 ml of tetrahydrofuran are now added to thegrey-brown solution at reflux temperature and the mixture is refluxedfor a further 90 minutes. The cooled solution is treated with 250 ml of5-N hydrochloric acid and extracted three times with ether. The combinedether phases are washed three times with water, dried over sodiumsulphate and concentrated, there being obtained 133.6 g (96% yield) of2,5-dimethyl-4-cyano-tetrahydrofuran-3-one of boiling point 109°-111°C./18 Torr. Gas-chromatographical identification (GC) on a 3 m column,2% Carbowax on Chromosorb: retention times of 2.9 and 3.2 minutes aremeasured at 200° C. (diastereomeric mixture); n₂₀ ^(D) =1.450.

(b) 7 g of the foregoing cyclic nitrile and 21 g of sodium bicarbonateare dissolved in 400 ml of water in a flask, treated at 10° C. with asolution of 25 g of Caroat (Degussa) in 80 ml of water and, after 30minutes, extracted four times with 150 ml of ethyl acetate each time.The combined organic phases are dried over sodium sulphate andconcentrated, there being obtained 4.3 g (55% yield) of a diastereomericmixture of 2,5-dimethyl-4-hydroxy-4-cyano-tetrahydrofuran-3-one ofboiling point ca 95° C./0.04 Torr; n₂₀ ^(D) =1.468; IR: 3400 nm(strong), 3010 nm, 2970 and 2910 nm (doublet), 2280 nm (weak), 1780 nm(medium), 1385 nm (strong), 1110 nm (strong).

(c₁) 5 g of the foregoing cyanohydrin and 7.5 g of ion exchanger Dowex 2(OH.sup.⊖ form) are refluxed in 30 ml of water for 1 hour and then themixture is filtered. The filtrate is saturated with sodium chloride andextracted four times with 80 ml of ethyl acetate each time. The combinedorganic phases, dried over sodium sulphate, are concentrated and give 2g (50% yield) of an oil which crystallises out upon standing. Identitywith 4-hydroxy-2,5-dimethyl-3(2H)-furanone is established by thin-layerchromatography and NMR.

(c₂) 1 g of the foregoing cyanohydrin and 0.7 g of triethylamine arerefluxed in 10 ml of toluene for 15 minutes. 5 g of Kieselgel (Merck)are now added and the mixture is filtered. The concentrated solutiongives 0.34 g of 4-hydroxy-2,5-dimethyl-3(2H)-furanone.

EXAMPLE 2

(a) 72.6 g (0.55 mol) of butyl glycolate are added dropwise to 12 g (0.5mol) of sodium hydride in 500 ml of tetrahydrofuran. The resultinggreen-brown mixture is treated at reflux temperature with 36.9 g ofcrotonic acid nitrile in 50 ml of tetrahydrofuran and the mixture isheld at reflux temperature for 90 minutes. The cooled mixture isadjusted to pH 9 with sodium bicarbonate solution and washed three timeswith ether. The aqueous phase is acidified to pH 1 with hydrochloricacid and extracted four times with ether. The dried and concentratedether phases give 29.1 g of 4-cyano-5-methyl-tetrahydrofuran-3-one ofboiling point 112°-116° C./13 Torr; GC (Carbowax, 180° C.) one peak; IR(film): 2250 (multiplet CN); 1783 (singlet, C═O).

(b) 3.1 g of the foregoing cyclic nitrile, 10.5 g of sodium bicarbonateand 2 g of sodium hydroxide are dissolved in 30 ml of water in a flaskand treated at 15°-20° C. with a solution of 11 g of Caroat (Degussa) in35 ml of water. After 30 minutes, the mixture is extracted five timeswith 35 ml of ethyl acetate each time and the combined ethyl acetatephases are dried and concentrated. There are obtained 2.2 g (63% oftheory) of 4-cyano-4-hydroxy-5-methyl-tetrahydrofuran-3-one in the formof a light-brown oil; GC: one peak; NMR (CDCl₃) shows complex multipletsbetween 3.3-4.7 ppm and 1-1.8 ppm as well as one OH signal at 5.7 ppm.

(c) 1.07 g of the resulting cyanohydrin and 2.0 g of sodium gluconateare refluxed in 15 ml of water for 15 minutes. The mixture is extractedfive times with ethyl acetate, the ethyl acetate phases are dried andconcentrated, there being obtained 0.42 g of4-hydroxy-5-methyl-3(2H)-furanone. The recrystallised material (meltingpoint 111°-120° C.) shows the following NMR (CDCl₃): 7.3 ppm (singlet1H, OH); 4.5-4.65 ppm (multiplet 3H, CH₃).

EXAMPLE 3

(a) 8.7 g (0.2 mol) of sodium hydride are suspended in 100 ml oftetrahydrofuran and the suspension is stirred at room temperature for 90minutes with 23.6 g (0.2 mol) of ethyl lactate. 9.5 g (0.18 mol) ofacrylonitrile are then added dropwise at 60° C. and the mixture isrefluxed for 90 minutes. The mixture is poured into 200 ml of water andwashed twice with 100 ml of ether each time. The aqueous phase isadjusted to pH 1 with 2-N hydrochloric acid and extracted three timeswith 150 ml of ether each time. The dried and concentrated ether phasesgive 20.1 g (89%) of 4-cyano-2-methyltetrahydrofuran-3-one of boilingpoint 116°-118° C./14 mmHg; IR: 2250 (CN), 1780 (C═O); NMR (CDCl₃): 1.37ppm doublet (CH₃); 3.4-4.9 ppm multiplet.

(b) The foregoing product is treated in a manner analogous to thatdescribed in Example 2 (b). There is obtained in 71% yield4-cyano-4-hydroxy-2-methyltetrahydrofuran-3-one in the form of thediastereomeric mixture; IR: 3350 (OH); 2290 weak (CN), 1785 and 1730(C═N); NMR (CDCl₃): 1.4-1.7 ppm multiplet (CH₃); 3.7-4.8 ppm multiplet(3×O--C--H), 7.1 singlet (OH).

(c) 3 g of the resulting cyanohydrin are dissolved together with 2.2 gof sodium acetate in 40 ml of water and the solution is heated to 70° C.for 15 minutes. The mixture is then extracted five times with 50 ml ofmethylene chloride each time. The dried and concentrated methylenechloride phases give 150 mg (6%) of crystalline4-hydroxy-5-methyl-3(2H)-furanone, which is identical with the productobtained according to Example 2 (c).

EXAMPLE 4

(a) 2.3 g (0.1 mol) of sodium are dissolved in 50 ml of isopropanol and,while cooling, the solution is treated with 14.5 g (0.11 mol) of ethyl2-hydroxybutyrate. 7.4 g (0.11 mol) of crotonic acid nitrile are addeddropwise at reflux temperature and subsequently the mixture is refluxedfor a further 90 minutes. The mixture is poured into 100 ml of water andwashed twice at pH 11 with 100 ml of methylene chloride each time. Theaqueous phase is adjusted to pH 1 with concentrated hydrochloric acidand extracted five times with 100 ml of methylene chloride each time.The combined organic phases are dried over sodium sulphate andconcentrated to give 10.2 g (66%) of2-ethyl-4-cyano-5-methyl-tetrahydrofuran-3-one (diastereomeric mixture)of boiling point 254° C.; IR: 2370 (C═N); 1775 (C═O); MS: 153; 138; 125;68 (100%).

(b) 6 g of the foregoing nitrile are dissolved together with 3.8 g ofborax and 3.2 g of sodium hydroxide in 40 ml of water and the solutionis treated portionwise with 17 g of Caroat. After 30 minutes isacidified with dilute sulphuric acid (1:1) and extracted four times with50 ml of ethyl acetate each time. After drying and concentration, thereare obtained 5.5 g (83%) of2-ethyl-4-cyano-4-hydroxy-5-methyl-tetrahydrofuran-3-one; IR: 3350 (OH),2240 weak (CN), 1780 and 1720 (C═O); NMR (CDCl₃): 0.8-2.2 ppm multiplet,3.5-4.9 ppm multiplet, 6.2 singlet (OH).

(c) 5 g of the product obtained according to the preceding paragraph aredissolved in 40 ml of water and the solution is adjusted to pH 1 with2-N sulphuric acid. After refluxing for 4 hours, the mixture is cooleddown and extracted five times with 50 ml of methylene chloride eachtime. The methylene chloride phases are dried over sodium sulphate andconcentrated to give 2.6 g (63%) of a mixture of2-ethyl-4-hydroxy-5-methyl-3(2H)-furanone and5-ethyl-4-hydroxy-2-methyl-3(2H)-furanone; MS: 142 (100%), 127, 114, 99,85, 71, 57, 4; IR: 3250 (OH), 1690 (C═O), 1615 strong (C═C).

EXAMPLE 5

(a) When there are used in Example 1 (a) in place of the crotonic acidnitrile 89.1 g (1.1 mol) of 2-pentenoic acid nitrile [prepared accordingto D. Mac Peek et al, J. Am. Chem. Soc. 81, 680 [1959]], there isobtained in 54% yield 5-ethyl-4-cyano-2-methyl-tetrahydrofuran-3-one;n_(D) ²⁰ =1.4552; IR: 2250 (CN), 1778 (C═O); MS: 153, 125, 96, 82(100%).

(b) The resulting nitrile is treated with Caroat in a manner analogousto that described in Example 1 (b) and gives in 83% yield viscous5-ethyl-4-cyano-4-hydroxy-2-methyl-tetrahydrofuran-3-one; n_(D) ²⁰=1.4587; IR: 3400 (OH), 2240 (weak, CN), 1785 (C═O); MS: 142 (M--HCN),114, 97, 82, 70 (100%).

(c) The cyanohydrin is treated in a manner analogous to that describedin Example 1 (c). There is obtained in 84% yield5-ethyl-4-hydroxy-2-methyl-3(2H)-furanone. n_(D) ²⁰ =1.5071. The esterisomerises partially upon standing [see Example 4 (c)].

EXAMPLE 6

(a) When, in Example 3 (a), the ethyl lactate is replaced by ethylα-hydroxybutyrate and the acrylonitrile is replaced by 2-pentenoic acidnitrile [prepared according to D. Mac Peek et al, J. Amer. Chem. Soc.81, 680 [1959]], then there is obtained, in addition to a small amountof dimeric pentenoic acid nitrile (which can be separated from the basicphase), in 50% yield 2,5-diethyl-4-cyanotetrahydrofuran-3-one; n_(D) ²⁰=1.4538; IR: 2250 (CN); 1775 (C═O). NMR (CDCl₃): 3.5-4.6 m/2 pr (H2,H5); 3.25 d and 3.08 d/1 pr (H4); 1.4-2.1 m/4 pr (2×CH₂); 0.8-1.3 m/6 pr(2×CH₃); MS: 167 (M⁺), 139, 82 (100%).

(b) The resulting nitrile is oxidised in a manner analogous to thatdescribed in Example 2 (b). There is obtained in 79% yield viscous2,5-diethyl-4-cyano-4-hydroxytetrahydrofuran-3-one; n_(D) ²⁰ =1.4588;IR: 3300 (OH) 2240 (weak, CN), 1782 (C═O); MS: 156 (M--HCN), 97, 82, 70(100%).

(c) The resulting cyanohydrin is cleaved in a manner analogous to thatdescribed in Example 3 (c). There is obtained in 75% yield2,5-diethyl-4-hydroxy-3(2H)-furanone of boiling point 50°-55° C./0.03mmHg. After recrystallisation from diisopropyl ether, the melting pointis 94°-96° C.; IR: 3250 (OH), 1690 (C═O), 1620 (C═C); NMR (CDCl₃): 7.3ppm s/1 pr (OH), 4.37 tr broad/1 pr (H2), 2.65 quart/2 pr (CH₂ at C5),1.5-2.2 m/2 pr (CH₂ at C2), 0.8-1.4 2×tr/6 pr (2×CH₃); MS: 156 (M⁺),141, 99, 71, 58 (100%).

EXAMPLE 7

(a) If, in Example 3 (a), the ethyl lactate is replaced by ethylα-hydroxybutyrate, then there is obtained in 68% yield2-ethyl-4-cyanotetrahydrofuran-3-one; n_(D) ²⁰ =1.4641; IR: 2260 (CN),1775 (C═O); MS: 139 (M⁺), 111 (M--CO), 107, 57, 54 (100%).

(b) The nitrile is oxidised in a manner analogous to that described inExample 2 (b). There is obtained in 50% yield2-ethyl-4-cyano-4-hydroxytetrahydrofuran-3-one in the form of a yellowoil; n_(D) ²⁰ =1.4586; IR: 3400 (OH), 2250 (weak, CN), 1783 (C═O); NMR(CDCl₃): 4.70 ppm d/1 pr (H5), 3.9-4.4 m/2 pr (H2 and OH), 3.88 d/1 pr(H5), 1.5-2.2 m/2 pr (CH₂ at C2), 1.05 tr/3 pr (CH₃).

(c) The resulting cyanohydrin is treated in a manner analogous to thatdescribed in Example 2 (c). There is obtained in 41% yield5-ethyl-4-hydroxy-3(2H)-furanone of melting point 47°-48° C. (fromdiisopropyl ether); IR (chloroform): 3250 (OH), 1710 (C═O), 1620 (C═C);NMR (CDCl₃): 6.4 ppm s broad/1 pr (OH), 4.53 s/2 pr (H2), 2.69 quart/2pr (CH₂ at C5), 1.27 tr/3 pr (CH₃).

What is claimed is:
 1. A process for the manufacture of compounds of thegeneral formula: ##STR8## wherein R represents a hydrogen atom or themethyl or ethyl group, which process comprises subjecting a compound ofthe general formula: ##STR9## wherein R has the significance givenearlier in this claim, to the cyanohydrin cleavage, by thermaltreatment, at temperatures of about 50° C. to about 400° C., or bytreatment with a base, at temperatures of about 50° C. to about 200° C.,in the gas phase or in the liquid phase.
 2. A process according to claim1, wherein a compound of the formula II is prepared by oxidizing acompound of the general formuls: ##STR10## wherein R has thesignificance given in claim 1, in a polar solvent, at a pH of about3-11, between about -10° C. and 60° C.
 3. A process according to claim2, wherein a caroate is used as the oxidizing agent.
 4. A processaccording to claim 1, wherein R is methyl.
 5. A process according toclaim 2, wherein R is methyl.
 6. A process according to claim 3, whereinR is methyl.